1. Field of the Invention
The invention relates to electric lamps having light source capsules with generally planar seals, and more particularly, to an improved support strap for these seals and an improved support frame incorporating these straps.
2. Description of the Prior Art
Electric lamps which have a light source capsule with a generally planar seal(s) include, among others, high intensity discharge (HID) metal halide and mercury vapor lamps. The light source capsule in these lamps is a discharge vessel of fused silica (quartz glass) which typically is sealed at both ends by a press seal which includes two major, substantially parallel faces and two minor, side faces extending between the major faces. Conductive lead-throughs extend through the press seal in a gas-tight manner to a pair of discharge electrodes arranged within the discharge vessel.
These lamps typically have an outer envelope which is sealed at one end by a lamp stem. A frame consisting of metallic support rods extends from the lamp stem and supports the discharge vessel within the outer envelope. Metallic support straps secured about the press seals are welded to a support rod on one or both sides of the press seal to secure the discharge vessel to the frame.
The pressing of hot fused silica produces significant variations in the resulting press seals in both width and thickness during high speed lamp manufacture. These dimensional variations present difficulties in achieving satisfactory strap designs. Many of the designs require hand fitting and adjusting of the straps on each discharge vessel during assembly of the frame to achieve a discharge vessel mount which is sufficiently rigid to pass the pre-shipment 30" drop test criteria which is common in the industry.
One strap design utilizes thin metal strips placed above and below the major faces of the press seal. The strips are placed, formed about the press seal and welded to the frame by an automatic assembly machine. The strips are thin and flexible so that when they are formed about the press seal they do not break or chip the seals. Such a strap is shown for example in U.S. Pat. No. 3,424,935 (Gungle et al). This strap design has the disadvantage that the automatic assembly machines which are required to place and form the strips around the pinch seals and to weld the strips to the support frame are very expensive to design and build, or purchase. Additionally, the straps of Gungle '935 are welded on both sides of the seals to respective support rods which extend adjacent to each minor seal face.
U.S. Pat. No. 2,830,210 (Jenne, Jr. et al.) discloses a single-piece support strap which is designed both for planar seals and for seals having a circular cross-section. The strap includes an expanded center portion which engages only the minor faces of a press seal. On either side of the expanded center portion is a doubled leg part, one of which is creased and the other which includes the free ends of the strap welded together. The doubled leg parts provide a clamping force on the minor faces of the seal. The strap does not include any portions which extend in contact with the major faces of the seal. The straps are also welded to support rods extending opposite both major seal faces.
U.S. Pat. No. 2,918,592 (Pomfrett et al.) discloses a single piece strap which bends around the press seal at one minor face thereof. Its ends are welded together adjacent the other minor face and to a single support rod extending along one side of the discharge vessel. One portion of the strap has a channel shape which provides rigidity and extends along a major face of the seal. The remaining portions of the strap are "soft" and bend around both minor faces and the opposing major face of the press seal. From the design, it is evident that the strap holds the press seal with a force solely determined by the tension with which the "soft" portion can be held against the strap when it is welded to the channel portion. To achieve an acceptable fit despite the normal lamp-to-lamp variations in press seal dimensions, such a strap would also require hand fitting on each seal or an expensive automatic assembly machine.
Additionally, a well-known characteristic of lamps which have a fused silica discharge vessel and an ionized plasma of alkali-halides during lamp operation, such as metal halide lamps, is the increase of lamp voltage that occurs over the lifetime of such lamps due to the diffusion of sodium ions through the heated discharge vessel. Sodium diffusion is accelerated by the presence of negative space charges on the outer surface of the discharge vessel. The negative space charges occur if ultraviolet radiation from the discharge strikes current carrying metal components within the lamp, which causes the production of photoelectrons.
In such lamps it is desirable to cover exposed metal parts with a material impervious to ultraviolet radiation and having a high photo electric work function, for example, as disclosed in U.S. Pat. Nos. 3,484,637 (Van Boort et al.) and 4,866,328 (Ramaiah et al). Another approach is to reduce the amount of metal in close proximity and in direct view of the discharge vessel, as in the above patent to Gungle which eliminates the elongate support rod extending adjacent the discharge vessel. However, the Gungle lamp still has a significant amount of metal parts since it includes two axially extending support rods connected to each of the support straps. Since ultraviolet radiation from the discharge vessel is also reflected off the inner surface of the outer envelope, these metal parts are still a source of a significant amount of photoelectrons.